One form of optical communications with increasing commercial importance utilizes passive sharing of an optical fiber among several optoelectronic sources and detectors (referred to hereinafter as "optical busing").
One specific example of optical busing is the "Passive Optical Network" (PON) illustrated in FIG. 1. Here, several terminal units (Optical Network Units--ONU) are linked by one or more passive optical couplers (POC) and optical fibers to a service provider Optical Subscriber Unit (OSU) that may in turn be the gateway to an external network. Data is transmitted within the network by either or both of time division multiplexing and wavelength division multiplexing.
In one currently favored implementation of a PON, the OSU is allocated a transmit mode 110 of approximately half of each cycle to transmit information while the ONUs "listen" in a receive mode. In the second half of each cycle, the ONUs are allocated individual time slots in which to transmit data 120 while the OSU in turn listens. The data burst signals transmitted by each ONU during one of these time slots (T1-TN) are referred to as a "packet".
Our U.S. Pat. No. 5,025,456, issued on Jun. 18, 1991, and our U.S. patent applications identified by Ser. Nos. 07/976037 and 07/976039, concurrently filed on Nov. 13, 1992, resolve several problems faced by electronic receiver circuits operating in such "burst-mode" packet communication systems.
Our U.S. patent '456describes a fundamental technique for dynamically establishing a logic threshold voltage centered between the extremes of burst-mode data signals, thus solving one well-known problem. The U.S. patent application Ser. No. 07/976037 introduces a precision peak detector reset technique for solving the problem of handling closely spaced data packets of widely varying amplitude. The U.S. patent application Ser. No. 07/976039 describes a technique for canceling out low-frequency signals due to background light on the optical bus, thus solving another problem.
In certain packet communication applications, it may be advantageous to superimpose on the bus the combination of a low-frequency signal channel along with the high-frequency packet data. For example, this low-frequency signal channel might be used for distance ranging or for communicating audio or terminal status information.
An additional difficulty is that the burst-mode packet data may have spectral energy in the same frequency band as the low-frequency signal. Yet, none of these communication channels is permitted to interfere with the proper detection of any other channel.
Thus, there is a need for a burst-mode packet data receiver which can properly detect the low-frequency signal channel along with the high-frequency packet data.